Protective proteins, functions biological systems

There is no life without cells and there are no cells without membranes. The cell membrane of animal cells is very important in that it protects the organelles and also keeps undesired particles from entering the cell body. Proteins span the membrane and touch both the inside and outside of the cell. Their function is to interact with molecules outside the cell which includes the ability to serve as protein receptors for hormones, to bind to other cells in wound healing and in the immune response, and to transport molecules into and out of the cell. What is outlined in these few sentences, however, is a very complex biological system which has been described only phenomenologically for a long time. 

A biological membranes system is typically formed by the combination of lipids and proteins. In eukaryotic cells, the plasma membrane, also referred to as the cell membrane, is a protective barrier which regulates what enters and leaves the cell. The endomembrane system is composed of different kinds of membranes which divide the cell into structural and functional compartments within a eukaryotic cell, such as the endoplasmic reticulum, Golgi apparatus, mitochondria, endosome and lysosome. Covalent modification of proteins with lipid anchors (protein lipidation) facilitates association of the lipidated proteins with particular membranes in eukaryotic cells. Protein lipidation is one of the most important protein post-translational modifications (PTMs). Studying lipidated protein function in vitro or in vivo is of vital importance in biological research. 

Many of the chemical properties of thiols stem from the fact that the sulfur atom of a thiol is oxidized easily to several higher oxidation states. The most common reaction of thiols in biological systems is their oxidation to disulfides, the functional group of which is a disulfide (—S—S —) bond. Thiols are readily oxidized to disulfides by molecular oxygen. In fact, they are so susceptible to oxidation that they must be protected from contact with air during storage. Disulfides, in turn, are easily reduced to thiols by several reagents. This easy interconversion between thiols and disulfides is very important in protein chemistry, as we will see in Chapter 18 ... 

Another factor peculiar to biological systems must be considered optical purity. Proteins are made of L-amino acids. As such, a chemical synthesis must start with L-amino acids and racemization must be minimized during the synthesis. This is especially true in the synthesis of an enzyme, as catalytic activity is dependent upon optical integrity. Amino acids are particularly susceptible to racemization once they have been acylated (i.e., addition of an acyl protecting group to the amino function) via intermediate azlactone... 

This introduction of substrates postpolymerization is normally utilized in polymer self-assembly to allow for the incorporation of incompatible or reactive functionalities. A number of synthetic methods can be utilized but the most facile is using the phase-separated nature of micelles or vesicles to selectively encapsulate a small molecule within a particular domain of the nanoparticle. This is often used to sequester hydrophobic small molecules within the core domain of the particle and hence provide protection from the surrounding aqueous environment. This is especially useful for the application of these nanoparticles as delivery vehicles for biological systems. Using reverse micelles hydrophilic molecules can also be selectively encapsulated within the nanoparticle. A recent example of effective encapsulation using a vesicular system was reported by Discher, who utilized poly(ethylene oxide)-fi-poly(butadi-ene) for the formation of amphiphilic diblocks that self-assembled in the presence of a number of proteins to form hollow polymeric particles with specific incorporation of the protein (Aranda-Espinoza et al., 2001 Bermundez et al., 2002). 

Deficiency can lead to cancer and immune system dysfunction Stabilizes proteins used in the immune system Promotes membrane binding that is essential for proper protein localization or biological function May hinder progression of adrenoleukodystrophy Reduces low-density lipoprotein in the blood Reduces platelet stickiness and protects against coronary heart disease... 

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